Known processes for producing a copper-clad laminate include a continuous double belt pressing process, in which a laminating material comprising a desired number of resin-impregnated bases (prepregs) in continuous lengths and a copper foil of continuous length superposed on at least one of the outer surfaces of the bases is heated under pressure between a pair of belts as disclosed in JP-A-61-10456 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
According to this technique, resin-impregnated bases and a copper foil(s) both at room temperature are supplied between a pair of belts and heated at a temperature of about 200.degree. C. under a pressure of 50 kg/cm.sup.2 or higher in a heating zone of the double belt press to cure the rein. The united laminate is then rapidly cooled under pressure in a cooling zone of the double belt press and delivered out of the double belt press.
Application of high pressure of 50 kg/cm.sup.2 or more in the heating zone has been an essential condition in order to obtain a laminate free from bubbles, etc. However, use of such a high pressure has been attended by various problems. For example, in cases where a fluid, e.g., air or oil, is used as a pressurizing medium, a special sealing device was necessary to prevent the medium from leaking, making the apparatus large-sized. Still yet, it was difficult to completely prevent leakage. On the other hand, when rollers are employed to exert the presure, a large number of rollers having a small diameter are needed from the standpoint of assuring surface smoothness. However, since the load imposed on the rollers sufficient for applying the desired pressure would be too high for their strength, some devices such as a back-up mechanism should be required to prevent breakage of the small diameter rollers.
In an attempt of overcoming the disadvantages associated with the above-stated pressurizing processes, it has ben proposed to use a molten metal as a pressurizing medium and the laminate is pressed by the medium in the form a solid formed by cooling the molten metal to a temperature below its melting point. This technique, however, presents another problem in handling of the solidified metal.
In the above-described conventional laminating process, the resin-impregnated bases and copper foils supplied have room temperature. Immediately after curing of the resin by heating under pressure,, the copper-clad laminate is rapidly cooled under pressure to a temperature less than the glass transition point of the resin, followed by delivery. The resin-impregnated bases, copper foils, and steel belts for double belt pressing differ in coefficient of thermal expansion from each other. Therefore, rapid heating in the heating zone sometimes causes wrinkling of the copper foil due to the difference in thermal expansion. In order to prevent this, it has been suggested to previously apply sufficient tension to each of the resin-impregnated bases and the copper foils, but such a device was still unsatisfactory. Even if wrinkling does not occur, rapid cooling of the cured copper-clad laminate to a temperature less than the glass transition point of the resin under pressure leaves a great internal stress within the resulting copper-clad laminate, which would lead to dimensional instability.